1. Field of the Invention
The present invention relates to packet communications systems which transmit data along a communications channel (from station "A" to station "B"). A feedback path is provided on which station "B" will inform station "A" when a "valid" packet of data is received. The "validity" of a received packet will be determined at station "B" by means of a digital error control coding (for example the Cyclic Redundancy Check code CRC-16 or CRC-CCITT, the popular 16-bit codes used in the United States and Europe respectively). The advantage of the present invention reside in the way digital error control coding, spread-spectrum modulation, interleaving, and feedback techniques are utilized such that for the existing in situ channel conditions a minimum amount of energy is used to transmit each packet of information by automatically compensating for changes in the conditions of the channel.
2. Description of the Prior Art
In the prior art, data communications systems having feedback use an Automatic Repeat Request (ARQ) scheme. The three common variants are the Stop-and-Wait ARQ, the Go-Back-N Continuous ARQ, and the Selective-Repeat Continuous ARQ; variations of these three common variants also exist. All current feedback error control techniques suffer a common disadvantage in that they discard an entire frame of information if an error is detected.
U.S. Pat. No. 3,754,211 issued on Aug. 21, 1973 to Edward Y. Rocher et al discloses a data communications system by which blocks are retransmitted when no acknowledgement is received within a specified period of time.
Japanese patent number 55-73157 published Feb. 6, 1980 by Kazumasa Kumakura and UK. Patent Application published Mar. 11, 1987 by Takahashi et al disclose the use of an ACK signal to acknowledge the proper receipt of data while a No ACK signal is sent in response to error corrupted data being received.
U.S. Pat. No. 4,803,685 issued Feb. 7, 1989 to Christian Oget and U.S. Pat. No. 4,905,234 issued Feb. 27, 1990 to Jeffrey S. Childress disclose a process to specify which frames have not been received properly and retransmitting those frames.
U.S. Pat. No. 4,393,501 issued Jul. 12, 1983 to Richard A. Kellogg et al and U.S. Pat. No. 4,270,205 issued May 26, 1981 disclose data communication systems with variable data rates.
The article by Mark Hewish et al (Defense Electronics & Computing Number 3 [Editorial Supplement to International Defense Review, July 1991]) discloses timing signals transmitted by the Global Positioning System (GPS) usable with the present invention.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the present invention as claimed.